Radially symmetrical animals lack a brain and nerve cord Why

Radially symmetrical animals lack a brain and nerve cord. Why?

Solution

Animals Symmetry:

The arrangement of body parts of animals around an axis are called as a symmetry. It is classified into 3 types

1.Asymmetrical: no symmetry (sponges)

2.Radial Symmetry: symmetry in the shape of a wheel; body parts arranged in a circle around a center axis (cnidarians, echinoderms)

3.Bilateral Symmetry: symmetry in which there is a right and left side that are mirror images.

Radial Symmetry:

The body parts of a radially symmetrical animal are arranged around a central axis so that each part extends from the center. Animals that exhibit radial symmetry tend to be sessile (immobile). Radial symmetry allows them to reach out in all directions.

Ex: Jellyfish and hydra

Jellyfish do not have a brain, a heart, a central nervous system, a skeletal system and also no bones and no blood. These have a simple nervous system called a nerve net (set of neurons). It move using a hydrostatic skeleton that controls the water pouch in their body to manipulate their movements.

Radially symmetrical animals lack a brain and nerve cord because of

An evolutionary process of environmental adaptation is the primary factor that causes absence of brain and nerve cord in radially symmetrical animals.

Echenoderms are descended from a bilaterally symmetrical ancestors that likely had some cephalization.

Present days Echenoderms lost the ancestral cephalization during the transition to radially symmetrical body plan.